[en] The synthesis of biopterin, D(and L)-erythro-neopterins, and their derivatives having a substituent at the 2-amino group via the 2-methylthio analogues is described. The 2-(methylthio)pteridines were synthesized by condensation of 4,5-diamino-6-hydroxy-2-(methylthio)pyrimidine with 5-deoxy-L-arabinose and D(and L)-arabinose phenylhydrazones, followed by oxidation with K₃[Fe(CN)₆] and O₂ in the presence of KI in an acidic solution, respectively. Aminolysis of these thioethers with ammonia and other amines in the presence of acetic acid gave the title compounds. (author)